This invention relates to a thin film semiconductor device for display and a method of producing same, and in particular to a thin film semiconductor device for display used in a large active matrix liquid crystal display with a built-in peripheral driving part and a method of producing same.
First, a common construction of an active matrix liquid crystal display device will be briefly explained, with reference to FIG. 1. As shown in FIG. 1, this active matrix liquid display device has a flat panel structure comprising a main substrate (101), a opposed substrate (102) and a spacer (103) affixing the main substrate (101) to the opposed substrate (102), and liquid crystal is held between the two substrates. On the surface of the main substrate are formed a display part (106) consisting of pixel electrodes (104) and switching devices (105) for driving the pixel electrodes (104) arranged in a matrix, and peripheral driving parts (107) connected to the display part (106). The switching devices (105) consist of thin film transistors. Thin film transistors are also formed in the peripheral driving parts (107) as circuit elements. The main substrate (101) having the above-described constitution will hereinafter be called a xe2x80x9cthin film semiconductor device for displayxe2x80x9d.
In the field of thin film transistors (TFT) integratedly formed in thin film semiconductor devices for display, structures using semiconductor films made from polysilicon are now being developed intensively and have been applied to relatively small (a few inches) active matrix liquid display devices. However, because polysilicon TFTs are made by high temperature processes, quartz substrates, which have excellent heat resistance, have been used for them. On the other hand, for comparatively large active matrix liquid crystal display panels (ten or so inches to several tens of inches) quartz substrates are not suitable due to their high cost and consequently glass substrates are employed. When a glass substrate is used, because its heat resistance is inferior, amorphous silicon TFTs, which can be made by relatively low temperature processes, have been employed. However, amorphous silicon TFTs have low mobility and P-channel amorphous silicon TFTs cannot be made. As a result it is impossible to form a peripheral driving apart on the glass substrate, and an externally attached driver device is used and mounted by the TAB method or the like. Consequently the number of pixels is restricted by the size of the screen and mounting limits. There therefore is a limit on how high-density thin film semiconductor devices for display using amorphous silicon TFTs can be made. Moreover, because an amorphous silicon TFT has low mobility, to obtain a sufficient ON-current the transistor size inevitably becomes large. Consequently the area of the display part occupied by the amorphous silicon TFTs for switching becomes large, and this is a hindrance to the realization of a high aperture ratio.
Recently, polysilicon TFTs with high mobilities which can be produced by low temperature processes have been being intensively developed. This technology involves converting an amorphous silicon film into a polysilicon film by locally heating the amorphous silicon film by annealing using an excimer laser. However, it is difficult for the processes other than the forming of the semiconductor films to be made low temperature processes and made performable with large substrates, and consequently this technology has not reached practical application. For example, one process which becomes problematic is that of forming the gate insulating layer. The gate insulating layers of current polysilicon TFTs are produced by thermally oxidizing polysilicon at approximately 1000xc2x0 C. When the above thermal oxidation is replaced by some other method in which the film-forming process is carried out at a low temperature, the gate insulating film lacks a sufficient withstand voltage. Also, in order to build the peripheral driving devices in, to simultaneously build N-channel TFTs and P-channel TFTs, ion implanting of an impurity has been carried out, but an ion implantation apparatus which can handle large substrates has not been realized, and difficult problems arise. Plasma vapor phase diffusion apparatuses for use in place of ion implantation apparatuses are now being developed, but impurity control is difficult and their practical use for mass production has not been realized. In addition to the above, the most difficult problem is that it has not been possible to produce TFTs having an LDD structure (hereinafter called LDD-TFTs) by low temperature processes and without using ion implantation. LDD-TFTs are indispensable as thin film transistors for switching, and are employed in small active matrix liquid crystal display devices to prevent pixel leakage. However, it is extremely difficult at present to form LDD-TFTs by low temperature processes and without using ion implantation.
In view of the problems associated with the technology discussed above, it is an object of the present invention to provide an LDD-TFT structure and manufacturing method with which large thin film semiconductor devices for display can be made by low temperature processes. A second object is, in making larger displays, to achieve increases in the performance of polysilicon TFTs included in peripheral driving parts while maintaining the LDD-TFT structure of display parts, in order to enable the incorporation of peripheral driving parts. A third object of the present invention is, in making larger displays, to provide a method by which it is possible to produce a black mask and a color filter of on-chip construction in order to obtain high pixel density and a high aperture ratio.
In order to solve the problems associated with the technology discussed above and achieve the objects of the invention, the following means were devised:
As the basic structure of the thin film semiconductor device for display of the present invention, there are provided a display part and a peripheral driving part formed integrally on a glass substrate. Pixel electrodes and thin film transistors for switching are arranged in a matrix in the display part. Thin film transistors to constitute circuit elements are formed in the peripheral driving part. Each thin film transistor is a bottom gate type comprising a gate electrode, a polycrystalline semiconductor layer formed on an insulating layer on the gate electrode, and a high concentration impurity film constituting a source and a drain formed on the polycrystalline semiconductor layer. The thin film transistors for switching are characterized in that they have an LDD structure wherein a low concentration impurity film is interposed between the polycrystalline semiconductor layer and the high concentration impurity film.
Preferably, the display part comprises an upper side part including the pixel electrodes, a lower side part including the thin film transistors for switching, and a color filter layer, a black mask layer and a planarization layer interposed between the two. In this case, the black mask layer contains a metal wiring pattern electrically connected to the high concentration impurity layer for the source and drain. Also, the pixel electrodes are electrically connected via the metal wiring pattern to the high concentration impurity film for the drain.
A film semiconductor device for display having the above-described constitution can be manufactured by the following low temperature processes: First, gate electrodes are formed on a glass substrate. Next, a semiconductor thin film is formed on an insulating film on the gate electrodes and then the semiconductor thin film is transformed into a polycrystalline layer by laser annealing. A low concentration impurity layer is then selectively formed only on the polycrystalline semiconductor layer included in the display part. Further, a high concentration impurity layer for sources and drains is formed on the low concentration impurity film, and thin film transistors for switching having a stacked LDD structure are thereby formed. At the same time, thin film transistors to be circuit elements are formed by directly forming a high concentration impurity layer for sources and drains on the polycrystalline semiconductor layer included in the peripheral driving part. Preferably, additional laser annealing is selectively performed on the high concentration impurity layers included in the peripheral driving part in order to reduce the resistance of the polycrystalline semiconductor layer.
According to this invention, after gate electrodes are formed on a glass substrate a semiconductor film is formed at low temperature on a gate insulating film on the gate electrodes. After that, the semiconductor thin film is transformed into a polycrystalline semiconductor layer by laser annealing. In this way it is possible to form a polycrystalline thin film transistor by low temperature processes. Because it is a bottom gate type, this structure does not readily suffer adverse influences from impurities such as sodium contained in the glass substrate. Also, because a polycrystalline semiconductor layer is used as the device region, it is possible to make the TFT small. In particular, in the thin film transistors for pixel switching, an LDD structure can be realized by forming a low concentration impurity layer and a high concentration impurity layer on the polycrystalline semiconductor layer by low temperature processes. In this way, pixel leakage and the like, which would be fatal defects in a display device, can be effectively prevented. In the thin film transistors constituting circuit elements of the peripheral driving part, on the other hand, N-channel TFTs and P-channel TFTs can be formed at the same time by superposing a high concentration impurity layer on the polycrystalline semiconductor layer by low temperature processes, and building-in of the driver is realized. At this time, additional laser annealing is selectively performed on the thin film transistors included in the peripheral driving part to increase the speed of these TFTs. In addition, by adopting an on-chip structure of a color filter layer, a black mask layer and a planarization layer, the invention contributes to higher pixel density and higher aperture rates.